Synthesis of gamma monoclinic sulfur and sulfur batteries containing monoclinic sulfur

ABSTRACT

The present invention relates to a method for making a novel cathode employing a monoclinic sulfur phase that enables a single plateau lithium-sulfur reaction in, for example, a carbonate electrolyte system. The cathode is applicable to a variety of other types of anodes. The method produces a cathode suitable for use in an electrode of a cell or battery by depositing monoclinic phase sulfur via vapor deposition onto a substrate in a sealed vapor deposition apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a bypass continuation of International applicationno. PCT/US20/13490, filed on Jan. 14, 2020, which, in turn, claims thebenefit of U.S. Provisional Application No. 62/792,068, filed on Jan.14, 2019, the entire disclosures of which are hereby incorporated byreference as if set forth fully herein.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract NumberNSF-1804374 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

State of the art lithium-sulfur (Li—S) batteries are attractivecandidates for use in hybrid electric vehicles (HEVs) and advancedportable electronic devices due to their order of magnitude highertheoretical energy density compared to conventional lithium-ionbatteries (LIB)¹⁻⁴. In addition, sulfur is both environmentally friendlyand naturally abundant in the earth's crust. However, the current Li—Ssystem is plagued by numerous challenges^(5,6). The insulating nature ofboth sulfur and the final discharge product, Li₂S results in lowmaterial utilization during the redox processes. A bigger challenge isthe dissolution of the intermediate reaction products,lithium-polysulfides (LiPs), into the electrolyte causing the well-known“shuttle-effect”. Polysulfide shuttle results in uncontrollabledeposition of sulfide species on the lithium metal anode, reducescoulombic efficiency, and increases capacity fade⁷. These challengeshave been extensively studied in the past decade with most studiesfocused on ether electrolyte-based Li—S batteries^(6,8,9).

A much less discussed, but extremely significant challenge forcommercial viability is the use of the ether electrolyte. Ether-basedsolvents are highly volatile and have low flash points posing asignificant risk of operating such batteries above roomtemperature¹⁰⁻¹². For example, dimethoxyethane (DME), an importantingredient used in present day Li—S batteries, has a boiling point ofonly 42° C. Therefore, the practicality of such Li—S battery chemistriesis doubtful due to severe safety concerns and transport issues¹³.

Lithium-ion batteries have been dominant in the commercial market forthe past 30 years, using carbonate-based electrolytes, well known fortheir reasonably safe behavior beyond room temperature (typical boilingpoints of >200° C.) and wide operational window^(14,15). In addition,flame retardant additives have been extensively researched, designed andapplied in carbonate-based electrolytes to better enhance theirreliability^(16,17). Hence, the knowledge gained on carbonateelectrolytes in the Li-ion battery field over the past three decades canpotentially be applied for the future development of Li—S batteries.However, it is known that when a carbonate electrolyte is used in Li—Sbatteries, an irreversible reaction between carbonate and polysulfidestakes place to form thiocarbonate and ethylene glycol, therebyterminating further redox reactions and shutting down the battery¹⁷. Ahandful of reports have recently demonstrated the use of Li—S batterieswith carbonate-based electrolytes with stable and reversiblecapacity¹⁸⁻²⁰. These papers propose a few different concepts/hypothesesthat potentially enable successful battery operation in carbonateelectrolytes. A common feature in these works is the nano-confinement ofsulfur.

For example, Xin et al. synthesized sulfur cathodes by confining sulfurmolecules into 0.5 nm pores of microporous carbon host materials¹⁹. Theyproposed that the confinement within the sub-nano pores prevented theformation of larger sulfur allotropes (S₅₋₈) and possibly resulted insmall sulfur allotropes (S₂₋₄) only, which in turn converted to Li₂Swithout the intermediate polysulfides (Li₂S₈, Li₂S₆, etc.). Theydemonstrated stable capacity (with a single discharge plateau) incarbonate electrolyte for up to 200 cycles. However, it is not clear howthe smaller allotropes exhibited a capacity close to the theoreticalcapacity of S₈→Li₂S conversion.

In another work, Fu et al synthesized carbon/sulfur cathodes with sulfurconfined in sub-nanometer carbon pores (0.4 nm-1 nm)²⁰. This materialexhibited a single plateau discharge and a stable reversible capacityfor 100 cycles in carbonate electrolyte. They proposed that the smallpore size forced the de-solvation of lithium ions and resulted insolid-state lithiation and de-lithiation of confined S₈ molecules.Overall, these works propose stringent pore size requirements (<0.5 nm)for the host carbon requiring complex synthesis procedures limitingbroad deployment, while also theoretically limiting the possible sulfurloading (due to limited available volume of precisely-sized micropores).Moreover, none of these reports attempt to characterize the initialsulfur allotropes (reactants) nor the discharge or charge productsformed and therefore the source of energy storage/capacity is unclear.

Commercially available ion-based batteries have reached their peak oftheoretical energy density. Thus, new specific capacity materials aresought for improving the energy density of ion-based batteries.Lithium-sulfur batteries have gained significant attention owing totheir promising high energy density (˜2600 Wh/kg) over the traditionalmetal-ion batteries (˜340 Wh/kg). Also, sulfur is a promising cathodematerial owing to its natural abundance, low adverse impact on theenvironment and low-cost.

Reduction of sulfur involves a multi-electron transfer redox reactionduring discharge and converts to lithium sulfide and vice-versa duringcharge, contributing to the higher capacity and thereby higher energydensity. However, state-of-the-art Li—S batteries employ ether-basedelectrolytes that face an array of challenges. Firstly, the intermediatedischarge products (polysulfides) formed in Li—S batteries that employthe ether-based electrolytes are highly soluble in ether-based solventsand can easily transport and shuttle between the cathode and anode. Thisphenomenon, the “shuttle effect,” results in loss of active sulfur andpassivation of Li metal leading to capacity (run time) fade withcycling. Secondly, ether-based solvents are highly volatile with lowflash points, thereby limiting battery application and posing asignificant risk for batteries operating at elevated temperatures.Finally, the use of the most common additive, LiNO₃ (needed forether-based Li—S batteries) is banned under U.S. transportation law dueto issues related to the formation of explosive gases. Therefore,despite the recent development of host materials to bind polysulfides inether-based Li—S batteries, the practical use of this electrolyte systemfaces significant operational and safety concerns.

The aforementioned issues of ether-based electrolytes can becircumvented by using a carbonate-based electrolyte. Carbonate-basedelectrolyte systems are used in used in commercial Li-ion batteries(LIBs) due to their safe and stable properties as well as their broadoperating temperature window. Also, a variety of high-temperature saltshave been investigated and designed for carbonate-based electrolytes forthe commercial Li-ion battery market to further enhance theirreliability.

However, the use of sulfur-based cathodes in combination withcarbonate-based electrolytes results in the irreversible formation ofundesirable by-products such as thiocarbonate and ethylene glycol, whichrender the battery non-functional. Recent developments in sulfurcathodes have improved systems employing carbonate-based electrolytesfor Li—S and other sulfur-based batteries using alternate anodes such asMg Na, and Ca.

Li—S batteries with carbonate electrolyte require confinement of thesulfur in nanopores. As a result, the complicated architecture requiredto implement these sulfur cathodes with the sulfur confined in thenanopores restricts their commercial application and reduces the sulfurloading of such batteries.

Gamma and Beta phases of sulfur are difficult to synthesize and aretypically metastable at room temperature. Current methods ofsynthesizing gamma phase sulfur are not entirely understood with only afew articles published in the past 150 years. When exposed to ambientair, gamma monoclinic phase sulfur is expected to convert back to astable orthorhombic structure, the most common phase of sulfur.

The use of commercially viable carbonate-based electrolytes forion-based sulfur batteries leads to the formation of irreversibleproducts (thiocarbonate and ethylene glycol) due to the reaction betweenpolysulfide species and electrolyte. Recently, strategies such asconfining smaller sulfur molecules (S₂-S₄) in microporous carbon (poresize<0.5 nm), crosslinking sulfur molecules to polymeric materials andformation of Solid Electrolyte Interphase (SEI) have led to use ofcarbonate-based electrolytes resulting in a solid to solid conversion.

Ambient temperature (about 18-25° C.) sodium-sulfur batteries (Na—S) arealso known in the art and are of interest due to their theoreticallyhigh energy density and low cost. Moreover, as compared to lithiumbatteries, Na—S batteries avoid the use of relatively expensive Li infavor of Na.

Lithium ion batteries comprise cathodes usually made from LiFePO₄ (LFP),LiMn₂O₄ (LMO), and LiNi_(x)Co_(y)Mn_(1xy)O₂ (NCM), and anode materialsthat may include Li₄Ti₅O₁₂ (LTO) and graphite (C). To test individualcapacities of the cathode (NCM) and anode (Graphite), they are coupledwith lithium and performance is evaluated. Further cathodes such as NCMand graphite are coupled to form a full cell. The voltage window ofLi∥NCM is 3-4 V and Li∥graphite is 0.6-0.01V and the coupled voltagewindow is 3-3.5 V. Hence, when sulfur cathodes are coupled withdifferent metal oxide/graphite anodes, the voltage window is typicallydifferent.

Ambient temperature sodium-batteries, potassium-batteries,magnesium-batteries, and calcium-batteries may comprise a cathodeincluding sulfur, an anode including sodium, potassium, magnesium, orcalcium, and an electrolyte. Generally, during the battery dischargecycle, polysulfides are reduced on the cathode surface and during thebattery charging cycle, polysulfides are formed at the cathode. Forexample:

2Na+xS↔Na₂S_(x)

Similar to i-S batteries, room temperature sodium-batteries faceproblems associated with achieving charge capacity close to theoreticaland retaining charge with repeated cycling. As such, commerciallyavailable Na—S batteries typically operate at temperatures of 300-350°C. so that they may employ molten sodium and sulfur phases inconjunction with a solid, ceramic electrolyte in order to mitigate theshuttle-effect.

Potassium-batteries, magnesium-batteries, and calcium-batteries posesimilar problems as sodium-batteries. Specifically, potassium-batteriesare larger and thus experience even more significant problems.

SUMMARY OF THE INVENTION

In a first aspect, the present invention relates to a method ofdepositing monoclinic sulfur on a substrate. In the method, monoclinicphase sulfur that is stable at a temperature below 80° C. is depositedvia vapor deposition onto a substrate which may be a porous material ina sealed vapor deposition apparatus at a temperature sufficient tovaporize the sulfur and for a time sufficient to provide the monoclinicphase sulfur, and the monoclinic phase sulfur deposited on the substrateis cooled to a temperature of 0-50° C. or 15-40° C. or 18-25° C.

In the method, a cathode suitable for use in an electrode of a cell orbattery may be produced.

In each of the foregoing embodiments, a loading of the monoclinic phasesulfur on the substrate of at least 0.05 mg/cm², or at least 0.5 mg/cm²,can be provided.

In the foregoing embodiment, the heating step may be carried out at atemperature of from 140° C. to about 350° C., or from 140° C. to 250°C., or from 150° C. to 210° C., or from 160° C. to 190° C., or at about175° C.

In each of the foregoing embodiments, the heating step may be carriedout for a period of at least about 1 minute, or from 1 minute to 1000hours, or 1 minute to 48 hours, or from 10 to 30 hours, or from 20 to 28hours, or for about 24 hours.

In each of the foregoing embodiments, the cooling step may be carriedout by exposing the vapor deposition apparatus or the sample to atemperature of equal to or less than 50° C., or from about −20° C. toabout 25° C. or from 0° C. to about 25° C., or from about 20° C. toabout 25° C., or from about 20° C. to 22° C.

In each of the foregoing embodiments, the vapor deposition apparatus maybe cooled immediately upon completion of the heating step.

In each of the foregoing embodiments, the substrate may be a porousmaterial having a pore volume of 10-95%, after the step of depositingthe monoclinic phase sulfur.

In each of the foregoing embodiments, the substrate may be conductive ornon-conductive. In each of the foregoing embodiments, the substrate mayinclude carbon nanofibers such as a mat of carbon nanofibers orfree-standing carbon nanofibers.

In each of the foregoing embodiments, the substrate may be located inthe vapor deposition apparatus with a gap between the surface of aliquid sulfur reservoir and the substrate such that the substrate doesnot come into contact with liquid sulfur during the vapor depositionstep.

In each of the foregoing embodiments of the method, the monoclinic phasesulfur may comprise a form of sulfur selected from the group consistingof monoclinic gamma phase sulfur; monoclinic sulfur that best matchesmonoclinic gamma phase sulfur using PDXL Version 2.8.4.0 IntegratedPowder Diffraction Software; monoclinic gamma phase sulfur oriented inthe k direction; and monoclinic sulfur that best matches monoclinicgamma phase sulfur oriented in the k direction using PDXL Version2.8.4.0 Integrated Powder Diffraction Software.

In a second aspect, the present invention relates to a cathode whichmay, for example, be prepared by any of the foregoing methods.

In another aspect, the present invention relates to a cathode includingmonoclinic phase sulfur that is stable at a temperature below 80° C.located on a substrate which may be a porous material. In thisembodiment, the monoclinic phase sulfur may optionally be deposited onthe substrate by vapor deposition.

In each of the foregoing embodiments, the monoclinic phase sulfur maycomprise a form of sulfur selected from the group consisting ofmonoclinic gamma phase sulfur; monoclinic sulfur that best matchesmonoclinic gamma phase sulfur using PDXL Version 2.8.4.0 IntegratedPowder Diffraction Software; monoclinic gamma phase sulfur oriented inthe k direction; and monoclinic sulfur that best matches monoclinicgamma phase sulfur oriented in the k direction using PDXL Version2.8.4.0 Integrated Powder Diffraction Software; monocliniccyclo-hexa-cyclo-deca sulfur and monoclinic sulfur that best matches tocyclo-decasulfur-cyclo-hexasulfur using the PDXL Version 2.8.4.0Integrated Powder Diffraction Software.

In each of the foregoing embodiments of the cathode, the substrate maybe conductive, non-conductive, and may comprise carbon nanofibers suchas a mat of carbon nanofibers or free-standing carbon nanofibers.

In each of the foregoing embodiments of the cathode, the cathode mayhave an initial discharge capacity in a range of from about 300 mAh/g toabout 2500 mAh/g, or from about 350 mAh/g to about 2000 mAh/g, or fromabout 375 mAh/g to about 1700 mAh/g, or from about 400 mAh/g to about1300 mAh/g, or from about 600 mAh/g to about 1100 mAh/g, or from about750 mAh/g to 900 mAh/g, or 800 mAh/g, based on a total weight of sulfurin the cathode.

In each of the foregoing embodiments of the cathode, the cathode mayinclude a cathode current collector.

In each of the foregoing embodiments of the cathode, the cathode mayhave a monoclinic phase sulfur loading in a range of at least 0.05mg/cm², or at least 0.5 mg/cm² from about 1.0 mg/cm² to about 150.0mg/cm² or from about 5.0 mg/cm² to about 125 mg/cm², or from about 10mg/cm² to about 115 mg/cm².

In each of the foregoing embodiments of the cathode, the substrate maybe a porous material having a pore volume of 10-95%, measured after themonoclinic phase sulfur is present on the cathode.

In another aspect, the invention relates to a sulfur cell including anyone of the foregoing embodiments of the cathode, an anode, and anelectrolyte. The electrolyte may be a carbonate electrolyte. Thecarbonate electrolyte may be selected from ethylene carbonate,dimethylcarbonate, methylethyl carbonate, diethylcarbonate, propylenecarbonate, vinylene carbonate, allyl ethyl carbonate, and mixturesthereof. Preferably, in each of the foregoing embodiments of the sulfurbattery, the carbonate electrolyte may be selected from ethylenecarbonate, diethyl carbonate, propylene carbonate and mixtures thereof.

In each of the foregoing embodiments of the cell, the anode may be anion reservoir including an active material selected from alkali metals,alkaline earth metals, transition metals, graphite, alloys, andcomposites. In some embodiments, the anode includes an active materialselected from lithium, sodium, potassium, magnesium, calcium, zinc,copper, titanium, nickel, cobalt, iron, or aluminum.

In each of the foregoing embodiments of the cell, the cell may be alithium-sulfur cell, a sodium-sulfur cell, a potassium-sulfur cell, amagnesium-sulfur cell, and a calcium-sulfur cell.

In each of the foregoing embodiments of the cell, the cell may exhibit asingle plateau discharge curve with a discharge capacity of from 300mAh/g to about 1700 mAh/g, or from about 400 mAh/g to about 1300 mAh/g,or from about 600 mAh/g to about 1100 mAh/g, or from about 750 mAh/g to900 mAh/g, or 800 mAh/g after 1000 cycles at a C/2 rate, all based on atotal weight of the sulfur in the cathode, or the cell exhibits a singleplateau discharge curve with a discharge capacity of from about 300mAh/g to about 3000 mAh/g, or from about 400 to about 2500 mAh/g after afirst cycle at a C/10 rate, all based on a total weight of the sulfur inthe cathode.

In each of the foregoing embodiments of the cell, the cell may have adischarge capacity of from about 100 mAh/g to about 1500 mAh/g, or fromabout 400 mAh/g to about 1200 mAh/g, or from about 600 mAh/g to about900 mAh/g, or from about 650 mAh/g to 750 mAh/g, after 1300 cycles at aC/2 rate, all based on a total weight of the sulfur in the cathode.

In each of the foregoing embodiments of the cell, the cell may be arechargeable cell.

In each of the foregoing embodiments of the cell, the cell may include acurrent collector.

In another aspect, the present invention relates to a sulfur batteryincluding a plurality of the sulfur cells as described in each of theforegoing embodiments.

In each of the embodiments of the sulfur battery, the sulfur battery mayinclude porous separator, where the separator is provided between ananodic section and a cathodic section of the battery. The porousseparator may be selected from a porous polypropylene membrane, cotton,nylon, polyesters, glass fiber, polyethylene, poly(tetrafluoroethylene,polyvinyl chloride, anodic aluminum oxide (AAO) and rubber.

The present invention provides both a novel material for use in thecathode of a sulfur cell or battery namely, a monoclinic phase sulfurthat is stable at a temperature below 80° C., such as a monoclinic gammaphase sulfur, and a novel approach that does not require confinement ofthe sulfur in such sulfur batteries. The monoclinic phase sulfur wassynthesized and deposited using vapor deposition on the surface of asubstrate which may be a porous material including carbon nanofiberssuch as a mat of carbon nanofibers or free-standing carbon nanofibers.

Free standing carbon nanofibers deposited with gamma monoclinic phasesulfur electrodes: (a) allow use of carbonate-based electrolytes, (b)yield a solid to solid transition and thereby avoiding polysulfideformation, (c) form a stabilized gamma phase that remains stable forlong periods in ambient air, (d) eliminate the need for the dead weightof binders and collectors, (e) maintain conductive carbon challenges forbetter electron pathways, and (f) provide excellent mechanical stabilityduring cycling. Further, the method of forming/depositing the gammamonoclinic phase sulfur can be employed for large-scale development ofgamma phase and its use in sulfur batteries.

Additional details and advantages of the disclosure will be set forth inpart in the description which follows, and/or may be learned by practiceof the disclosure. The details and advantages of the disclosure may berealized and attained by means of the elements and combinationsparticularly pointed out in the appended claims. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the disclosure, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of the fabrication procedure for making carbonnanofibers (CNFs) and vapor deposition of gamma monoclinic phase sulfuron the CNFs.

FIG. 2 shows a vapor deposition assembly and its respective partssuitable for use in the method of the present invention.

FIGS. 3A-3D show scanning electron microscope (SEM) images of the vapordeposited gamma sulfur phase on CNFs FIG. 4A shows the X-ray diffractionpattern of vapor deposited sulfur on CNFs.

FIG. 4B shows a thermogravimetric analysis (TGA) curve of gammamonoclinic phase sulfur in a N₂ environment.

FIG. 5A shows Charge-Discharge curves at C/50 cycling of an Li—S cell.

FIG. 5B shows Charge-Discharge curves at C/2, C/5 and C/10 cycling ratesof the Li—S cell.

FIG. 5C shows Charge-Discharge curves at C/2 at various numbers ofcycles of the Li—S cell.

FIG. 5D shows cyclic voltammetry curves for the first, second and thirdcycles at 0.05 mV/s of the Li—S cell.

FIG. 5E shows the rate capability of the Li—S cell.

FIG. 5F shows C/2 cycling of an Li—S cell with vapor deposited gammamonoclinic phase sulfur on CNFs.

FIGS. 6A-6C show diagrams of the vapor deposition assembly.

FIG. 6A is a perspective view of the vapor deposition assembly,including the autoclave lid and autoclave chamber.

FIG. 6B is a cross-sectional view of the vapor deposition assembly ofFIG. 6A, taken along line A-A of FIG. 6A.

FIG. 6C is cross-sectional view of the vapor deposition assembly of FIG.6A, taken along line B-B of FIG. 6A.

FIG. 7 is a schematic of a lithium-sulfur battery discharge in thecarbonate-based electrolyte. At least some of the gamma-monoclinicsulfur is deposited on the external surface of the carbon nanofibers.The medium sized balls signify surface deposited gamma-monoclinicsulfur, and the small balls signify lithium sulfide, the product formedafter reduction of sulfur.

FIGS. 8A-8F show the material characterization of CNFs and γ-sulfurdeposited CNFs.

FIG. 8A shows SEM images of CNFs before sulfur deposition.

FIG. 8B shows SEM images of CNFs after gamma sulfur deposition. Theinset is an enlarged image of well-deposited gamma sulfur particles ofCNFs.

FIG. 8C shows a cross-sectional SEM image of CNFs after sulfurdeposition showing gamma sulfur deposited throughout the cathode.

FIG. 8D shows the pore size distribution of CNFs before and after gammasulfur deposition.

FIG. 8E shows isotherms obtained before and after gamma sulfurdeposition on CNFs.

FIG. 8F shows a TGA curve of gamma sulfur deposited CNFs in an argonenvironment.

FIGS. 9A-9F show the phase and surface characterization of CNFs andgamma sulfur deposited CNFs.

FIG. 9A shows an X-ray diffraction (XRD) pattern of CNFs and gammasulfur deposited CNFs.

FIGS. 9B-9C show SEM images of gamma sulfur deposited CNFs showingwell-distributed gamma sulfur deposition and EDS elemental mapping.

FIGS. 9D-9F show the X-ray photoelectron spectroscopy (XPS) spectra ofS, O, and C on the gamma sulfur deposited CNFs (after deposition ofSulfur).

FIGS. 10A-10F show the electrochemical characterization of gamma sulfurdeposited CNFs.

FIG. 10A shows charge-discharge patterns of gamma sulfur deposited CNFsin and ether electrolyte, 1,2-dimethoxyethane:1,3-dioxolane (DME:DOL)and a carbonate electrolyte, ethylene carbonate:diethylcarbonate (EC:DEC).

FIG. 10B shows cyclic voltammetry curves of gamma sulfur deposited CNFsin an ether electrolyte (DME:DOL) and a carbonate electrolyte (EC:DEC).

FIG. 10C shows the cycling stability of gamma sulfur deposited CNFs inan EC:DEC electrolyte at a current rate of 0.5 C.

FIG. 10D shows the discharge capacity of gamma sulfur deposited CNFs atvarious numbers of cycles.

FIG. 10E shows a differential capacity analysis plot of gamma sulfurdeposited CNFs displaying a single peak in the charge-discharge cycle.

FIG. 10F shows a Nyquist plot of gamma sulfur deposited CNFs cathode asa function of voltage during the charge-discharge cycle.

FIGS. 11A-11D show the rate performance and high loading analysis ofgamma sulfur deposited CNFs.

FIG. 11A shows the rate performance of gamma sulfur deposited CNFs incarbonate electrolyte.

FIG. 11B shows the discharge profiles of gamma sulfur deposited CNFs atvarious C rates.

FIG. 11C shows long cycling of gamma sulfur deposited CNFs at lowcurrent rate 0.1 C at 1.2 mg/cm² loading.

FIG. 11D shows long cycling of gamma sulfur deposited CNFs as a functionof high loading.

FIGS. 12A-12B show post-mortem SEM and TEM analysis of gamma sulfurdeposited CNFs after 20 Charge-Discharge Cycles.

FIG. 12A shows a SEM image of gamma sulfur deposited CNFs after 20discharge cycles.

FIG. 12B shows a SEM image of gamma sulfur deposited CNFs after 20charge cycles.

FIG. 12C shows a SEM image taken after lithiation in a completelydischarged sample post 1000 cycles, showing the adherence of sulfur tocarbon nanofibers.

FIGS. 13A-13G show post-mortem XRD and XPS analyses of gamma sulfurdeposited CNFs after five charge-discharge cycles, at C/10.

FIG. 13A shows an XRD pattern of pristine gamma sulfur deposited CNFs.

FIG. 13B shows an XRD pattern of discharged gamma sulfur deposited CNFsshowing conversion to Li₂S.

FIG. 13C shows a charged spectrum of a battery that was charged andsubsequently the electrode was removed from the battery, showingconversion to monoclinic cyclo-hexa-cyclo-deca sulfur.

FIG. 13D shows a typical charge-discharge voltage profile obtained inthe samples and the specific points where spectroscopy data wascollected.

FIG. 13E shows high resolution S2p XPS spectra of pristine gamma sulfurdeposited CNFs.

FIG. 13F shows high resolution S2p XPS spectra of discharged gammasulfur deposited CNFs displaying an Li₂S peak.

FIG. 13G shows high resolution S2p XPS spectra of charged gamma sulfurdeposited CNFs showing the presence of a sulfur peak.

FIGS. 13H and 13I show XPS spectra of discharged and charged samples offluorine, respectively. The formation of LiF layers on the surface andsome salt species indexed were observed at −685.5 eV ad 688 eV,respectively. The formation of LiF was not consistent on the surface asobserved in cycled SEM images.

FIG. 14A shows cyclic voltammetry (CV) curves of a gamma sulfurdeposited CNFs cathode at various can rates.

FIG. 14B shows the rate performance of a gamma sulfur deposited CNFscathode at various current densities.

FIG. 14C shows the cycling stability of cathodes at a 0.1 C rate.

FIG. 14D shows the post mortem XRD patterns of a gamma sulfur depositedCNFs cathode.

FIG. 15A shows CV curves of a gamma sulfur deposited CNFs cathode at 0.1mVs⁻¹ scan rates.

FIG. 15B shows the rate performance of a gamma sulfur deposited CNFscathode at various current densities.

FIG. 15C shows the cycling stability of a gamma sulfur deposited CNFscathode at a 0.5 C rate.

FIG. 16 is a schematic of a sulfur battery of the present inventionincluding anode and cathode current collectors and a porous separator.

FIG. 17 shows a survey spectra of gamma sulfur deposited CNFs, whichshows the existence of C1s, S2p, and O1s peaks in the composite.

FIG. 18A shows the cycling stability of a carbonate electrolyte at 0.1 Crate.

FIG. 18B shows the cycling stability of a carbonate electrolyte at 0.5 Crate.

FIG. 19 shows the cycling stability of an ether electrolyte.

FIGS. 20A-2B show comparisons of:

1. pure alpha orthorhombic sulfur,

2. monoclinic gamma phase sulfur of the invention, and

3. the gamma sulfur.cif file from the National Institute for MaterialsScience (NIMS) Materials Database discussed in Watanabe, Yasunari, “TheCrystal Structure of Monoclinic γ-Sulphur,” Acta Cryst. (1974), B30,1396, The NIMS Material Database can be found athttps://mits.nims.go.jp/index_en.html).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, a rare phase of sulfur—the monoclinic gammaphase sulfur (Rosykite)—is provided in a stable form on a support. Thisenables successful operation of sulfur cells and batteries in acarbonate electrolyte for up to or more than 4000 cycles.Electrochemical characterizations via galvanostatic charge-dischargetests, cyclic voltammetry, differential capacity and in-operando EISmeasurements suggest a redox mechanism that consistently eliminatesintermediate polysulfides over the entire 2000 cycles, thus preventingadverse reactions with the electrolyte.

Thus, in one embodiment, the present invention relates to a cathode thatincludes monoclinic gamma phase sulfur that is stable at temperatures upto 80° C., or at temperatures of from 0° C. to 80° C., or from 15° C. to50° C., or from 18° C. to 40° C., The monoclinic gamma phase sulfur canbe identified by a best match to PDF Card No.: 00-013-0141 Quality:B forRosickyite, monoclinic gamma phase sulfur, optionally using the PDXLVersion 2.8.4.0 (Integrated Powder Diffraction Software by Rigaku)database of XRD patterns to match the XRD pattern for this phase ofsulfur. The monoclinic gamma phase sulfur can also be identified by abest match to monoclinic gamma phase sulfur using the PDXL Version2.8.4.0 (Integrated Powder Diffraction Software by Rigaku) database ofXRD patterns to match the XRD pattern for this phase of sulfur.

The XRD patterns of FIGS. 20A-20B show comparisons of:

1. pure alpha orthorhombic sulfur,

2. monoclinic gamma phase sulfur of the invention, an

3. the gamma sulfur.cif file from the National Institute for MaterialsScience (NIMS) Materials Database discussed in Watanabe, Yasunari, “TheCrystal Structure of Monoclinic γ-Sulphur,” Acta Cryst. (1974), B30,1396, The NIMS Material Database can be found athttps://mits.nims.go.jp/index_en.html). In FIGS. 20A-20B peaks ofmonoclinic gamma phase sulfur seen at 13.59+/−0.02 match exactly with020 (hkl) peaks of monoclinic gamma phase sulfur from the database.Alpha phase sulfur has no peaks in this region. Similarly, themonoclinic gamma phase sulfur has a peak at 040 (hkl) which does notover-lap with orthorhombic sulfur, further suggesting preferentialorientation of the monoclinic gamma phase sulfur in the k direction.

In addition, none of the vapor deposited monoclinic gamma phase sulfurpeaks match with the alpha orthorhombic phase. Also, in the database,the monoclinic gamma phase sulfur peaks do not overlap with any alphaorthorhombic phase. This supports a conclusion that the peaks of gammasulfur are unique in the hkl direction (010), (020), (030), (040) . . .(090) when compared to the crystal structure database.

Further when the database gamma monoclinic phase is compared with thebeta monoclinic phase none of the peaks and their correspondingintensities match with each other.

Post mortem studies of cycled cells using XRD, XPS and TEM found redoxproducts after charge and discharge cycles including clear evidence forthe formation of Li₂S at the end of the discharge cycle. In addition,these studies indicate that the sulfur rearranges itself to yet anotheruncommon phase—cyclo-deca-cyclo-hexa sulfur, also belonging to themonoclinic crystal structure family, after the first charge.Nevertheless, no effect of such phase shift is seen on theelectrochemical redox behavior as shown by galvanostaticcharge-discharge tests. This seems to be the first report of these typesof stable sulfur crystal structures in Li—S cells or batteries and theiroperation in carbonate electrolyte.

The cyclo=deca-cyclo-hexa monoclinic phase sulfur may be identified by abest match to PDF Card No.: 01-072-4584 Quality:S forcyclo-decasulfur-cyclo-hexasulfur, optionally using the PDXL Version2.8.4.0 (Integrated Powder Diffraction Software by Rigaku) database ofXRD patterns to match the XRD pattern for this phase of sulfur.Alternatively, the cyclo=deca-cyclo-hexa monoclinic phase sulfur may beidentified by a best match to cyclo-decasulfur-cyclo-hexasulfur, usingthe PDXL Version 2.8.4.0 (Integrated Powder Diffraction Software byRigaku) database of XRD patterns to match the XRD pattern for this phaseof sulfur.

By “best match” in the context of using the software to match a PDF cardis meant a type of best match algorithm which determines the best matchof a particular XRD patent to one of the PDF cards for forms of sulfur.Thus, as demonstrated below, not all peaks of the XRD patent must matchthe pattern on the PDF card and not all peaks on the PDF care must bepresent for there to be a match. Rather, if the specified softwaredetermines that the XRD pattern of a particular material is a best matchto the specified PDF card, then the sulfur for which the XRD pattern hasbeen obtained is considered to be within the scope of the presentinvention.

As seen from the SEM images, FIGS. 3A-3D, 8B, 9B-9C, and 12A-12C showthe that the gamma monoclinic phase sulfur is non-confined as it showsblocks of sulfur in-between the interfiber carbon-nanofiber spacing. TheXRD data shows the presence of crystalline peaks denoting the presenceof gamma monoclinic phase sulfur on CNFs. See FIGS. 9a , 13A-13B, and14D. The present invention employs a porous material thus, a partialdeposition of sulfur might take place in the pores. Current methodsemploying carbonate electrolyte are only effective when sulfur isconfined, in contrast, the electrodes of the present invention arecapable of working despite being non-confined. The inventors theorizethat this is due to gamma monoclinic gamma phase of sulfur.

In the present invention, gamma monoclinic phase sulfur issynthesized/deposited on a support for a cathode of a sulfur cell.Despite an exposed un-confined deposition of this sulfur phase on asubstrate, carbonate-based cells employing this sulfur phase exhibit ahigh reversible capacity of, for example, 700 mAh/g after 2000 cycles,or 550 mAh/g after 4000 cycles, both at C/2 rate, or 550 mAh/g after 800cycles at C/10. Li—S electrodes with this sulfur phase also exhibit ahigh rate performance up to a 40 C rate and a high areal loading ofsulfur of up to 5 mg/cm². The electrode may consist of freestanding,binder and current collector-free carbon nanofibers. After sulfurdeposition and slow cooling at room temperature in an autoclave, thesulfur adopts the uncommon monoclinic gamma phase structure rather thanthe typical orthorhombic alpha phase, even on the external surface of asubstrate such as carbon nanofibers. This monoclinic gamma phase remainsstable at room temperature for over a year with no apparent evidence ofa phase change even beyond this timeframe. Extensive electrochemicalcharacterization and post mortem spectroscopy/microscopy studies oncycled cells reveal an altered redox mechanism that reversibly convertsmonoclinic sulfur to Li₂S without the formation of intermediatepolysulfides over up to and beyond 2000 cycles. The development ofnon-confined high loading sulfur cathodes for use in batteries employingcarbonate-based electrolytes can revolutionize the field of high energydensity practical batteries.

The present invention employs a monoclinic gamma phase sulfur thatenables a single plateau lithium-sulfur reaction in, for example, acarbonate electrolyte system. This system avoids the formation ofpolysulfides during charge-discharge. As a result, it is expected thatthe monoclinic gamma phase sulfur will have a long cycle life in abattery, even if a carbonate electrolyte system is employed.

Method for Preparing a Cathode Via Vapor Deposition

In one embodiment, the gamma monoclinic phase sulfur may be deposited ona substrate via vapor deposition. Vapor deposition is carried out in asealed vapor deposition apparatus at a temperature sufficient tovaporize sulfur and for a time sufficient to provide gamma monoclinicphase sulfur. Once the gamma monoclinic phase sulfur is deposited on thesubstrate, it is cooled to ambient temperature of about 18-25° C.Preferably, the sulfur loading on the substrate is at least 0.05 mg/cm².For this purpose, it may be desirable to employ a porous substrate inorder to increase the surface area of the substrate available for sulfurdeposition.

Suitable temperatures for carrying out the vapor deposition may rangefrom about 140° C. to 350° C., or from 140° C. to 250° C., or from 150°C. to 210° C., or from 160° C. to 190° C., or at about 175° C.Preferably, the vapor deposition is carried out for a period of at leastabout 1 minute, or from 1 minute to 1000 hours, or 1 minute to 48 hours,or from 10 to 30 hours, or from 20 to 28 hours, or for about 24 hours.

The vapor deposition step is followed by a step of cooling, which may becarried out by exposing the vapor deposition apparatus or the sample toa temperature of less than 50° C., or an environmental temperature ofless than 25° C., or from about −20° C. to about 25° C. or from 0° C. toabout 25° C., or from about 20° C. to about 25° C., or from about 20° C.to 22° C. In some embodiments, the vapor deposition apparatus is cooledimmediately upon completion of the vapor deposition step.

The porous material employed for the substrate in the present inventionmay be conductive or non-conductive, and preferably is selected from asubstrate comprising carbon nanofibers, a mat of carbon nanofibers,free-standing carbon nanofibers, conductive carbon powders, carbonnanotubes, graphene, and carbide-derived carbons. After the step ofdepositing the gamma monoclinic phase sulfur, the porous material mayhave a pore volume of 10-95%. Furthermore, during the method andparticularly the vapor deposition step, the porous material ispreferably located in the vapor deposition apparatus such that theporous material does not contact with the liquid sulfur.

The Cathode

The cathode of the present invention may be prepared by the method ofdepositing gamma monoclinic phase sulfur via vapor deposition onto asubstrate as described above. The pristine cathode includes monoclinicgamma monoclinic phase sulfur on a substrate. The gamma monoclinic phasesulfur loading on the cathode may be in a range of from about 1.0 mg/cm²to about 150 mg/cm² or from about 5 mg/cm² to about 125 mg/cm² or fromabout 10 mg/cm² to about 115 mg/cm², or at least from about 25 mg/cm².

The cathode may have an initial discharge capacity in a range of fromabout 300 mAh/g to about 2500 mAh/g, or from about 350 mAh/g to about2000 mAh/g, or from about 375 mAh/g to about 1700 mAh/g, or from about400 mAh/g to about 1300 mAh/g, or from about 600 mAh/g to about 1100mAh/g, or from about 750 mAh/g to 900 mAh/g, or 800 mAh/g, based on atotal weight of the sulfur in the cathode.

The cathode of the present invention may optionally include a cathodecurrent collector though a current collector will not typically berequired.

The Anode

The anodes of the present invention are ion reservoirs, optionallyincluding an active material selected from alkali metals, alkalinemetals, transition metals, graphite, alloys, and compositions.

Suitable examples of active materials may be selected from lithium,sodium, potassium, magnesium, calcium, zinc, copper, titanium, nickel,cobalt, iron, aluminum, silicon, germanium, tin, lead, antimony,bismuth, manganese, and cadmium, and lithiated versions thereof. Theactive materials of the anodes may also be alloys or intermetallicselected from compounds of lithium, sodium, potassium, magnesium,calcium, zinc, copper, titanium, nickel, cobalt, iron, aluminum,silicon, germanium, tin, lead, antimony, bismuth, manganese, andcadmium, and lithiated versions thereof, where the alloys or compoundsare stoichiometric or non-stoichiometric.

In some embodiments the active materials of the anode may be oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides oflithium, sodium, potassium, magnesium, calcium, zinc, copper, titanium,nickel, cobalt, iron, aluminum, silicon, germanium, tin, lead, antimony,bismuth, manganese, and cadmium, and their mixtures or composites, andlithiated versions thereof.

In some embodiments, the active materials of the anodes may be salts andhydroxides of selenides and lithiated versions thereof, or carbon orgraphite materials and prelithiated versions thereof; and combinationsthereof.

Preferably, the anode in the sulfur cells may include an active materialselected from lithium, sodium, potassium, magnesium, calcium, zinc,copper, titanium, nickel, cobalt, iron, or aluminum. Preferably, thecell is a lithium-sulfur cell, a sodium-sulfur cell, a potassium-sulfurcell, a magnesium-sulfur cell, and a calcium-sulfur cell, and even morepreferably, the cell is selected from a lithium-sulfur cell,sodium-sulfur cell, and a potassium-sulfur cell.

The Cells

The sulfur cells of the present invention include the cathodes of thepresent invention in combination with an anode.

The cells of the present invention may exhibit a single plateaudischarge curve with a discharge capacity of from 300 mAh/g to about1700 mAh/g, or from about 400 mAh/g to about 1300 mAh/g, or from about600 mAh/g to about 1100 mAh/g, or from about 750 mAh/g to 900 mAh/g, or800 mAh/g after 1000 cycles at a C/2 rate, all based on a total weightof the sulfur in the cathode.

The cells of the present invention may also exhibit a single plateaudischarge curve with a discharge capacity of from about 300 mAh/g toabout 3000 mAh/g, or from about 400 to about 2500 mAh/g after a firstcycle at a C/10 rate, all based on a total weight of the sulfur in thecathode.

The cells of the present invention also have a discharge capacity offrom about 100 mAh/g to about 1500 mAh/g, or from about 400 mAh/g toabout 1200 mAh/g, or from about 600 mAh/g to about 900 mAh/g, or fromabout 650 mAh/g to 750 mAh/g, after 1300 cycles at a C/2 rate, all basedon a total weight of the sulfur in the cathode.

The cell may also include an electrolyte. Suitable electrolytes for usein the cell of the present invention are carbonate electrolytes andether electrolytes. Preferably, the electrolytes are carbonateelectrolytes and the carbonate electrolytes may be selected ethylenecarbonate, dimethylcarbonate, methylethyl carbonate, diethylcarbonate,propylene carbonate, vinylene carbonate, allyl ethyl carbonate, andmixtures thereof. More preferably, the carbonate electrolyte is selectedfrom ethylene carbonate, diethyl carbonate, propylene carbonate andmixtures thereof.

Generally, the anodes employed in the sulfur cell is an ion reservoirincluding an active material selected from alkali metals, alkaline earthmetals, transition metals, graphite, alloys, and composites. In someembodiments, the anode has an active material selected from lithium,sodium, potassium, magnesium, zinc, copper, titanium, nickel, cobalt,iron, calcium, or aluminum. Preferably, the battery is a lithium-sulfurcell, a sodium-sulfur cell, a magnesium-sulfur cell, a potassium-sulfurcell and a calcium-sulfur cell, and even more preferably, the battery isselected from a lithium-sulfur cell, sodium-sulfur cell, and apotassium-sulfur cell.

The cell is preferably a rechargeable cell. The cell may optionallyinclude one or both of a current collector and a cathode currentcollector.

Batteries

The sulfur battery of the present invention includes a plurality of thesulfur cells according to the present invention.

Suitable electrolytes to be used in the battery of the present inventionare carbonate electrolytes. Preferably, the carbonate electrolyte isselected from ethylene carbonate, dimethylcarbonate, methylethylcarbonate, diethylcarbonate, propylene carbonate, vinylene carbonate,allyl ethyl carbonate, and mixtures thereof. More preferably, thecarbonate electrolyte is selected from ethylene carbonate, diethylcarbonate, propylene carbonate and mixtures thereof.

Generally, the anodes employed in the sulfur battery is an ion reservoirincluding an active material selected from alkali metals, alkaline earthmetals, transition metals, graphite, alloys, and composites. In someembodiments, the anode includes an active material selected fromlithium, sodium, potassium, magnesium, zinc, copper, titanium, nickel,cobalt, iron, calcium, or aluminum. Preferably, the battery is alithium-sulfur battery, a sodium-sulfur battery, a magnesium-sulfurbattery, a potassium-sulfur battery and a calcium-sulfur battery, andeven more preferably, the battery is selected from a lithium-sulfurbattery, sodium-sulfur battery, and a potassium-sulfur battery.

The batteries of the present invention exhibit a single plateaudischarge curve with a discharge capacity of from 300 mAh/g to about1700 mAh/g, or from about 400 mAh/g to about 1300 mAh/g, or from about600 mAh/g to about 1100 mAh/g, or from about 750 mAh/g to 900 mAh/g, or800 mAh/g after 1000 cycles at a C/2 rate, all based on a total weightof the sulfur in the cathode.

The batteries of the present invention may also exhibit a single plateaudischarge curve with a discharge capacity of from about 300 mAh/g toabout 3000 mAh/g, or from about 400 to about 2500 mAh/g after a firstcycle at a C/10 rate, all based on a total weight of the sulfur in thecathode.

The batteries of the present invention also have a discharge capacity offrom about 100 mAh/g to about 1500 mAh/g, or from about 400 mAh/g toabout 1200 mAh/g, or from about 600 mAh/g to about 900 mAh/g, or fromabout 650 mAh/g to 750 mAh/g, after 1300 cycles at a C/2 rate, all basedon a total weight of the sulfur in the cathode.

The batteries of the present invention have the advantage that they arecapable of operating below 250° C., more preferably, below 150° C., orbelow 100° C. or bellow 50° C. or below 25° C.

Preferably, the battery is a rechargeable battery. The battery describedherein may optionally include one or both of a current collector and acathode current collector.

The battery of the present invention may further include a separator,such as a porous separator, provided between an anodic section and acathodic section of the battery. Preferably, the porous separator is aporous material, such as a polypropylene membrane, cotton, nylon,polyesters, glass fiber, polyethylene, poly(tetrafluoroethylene),polyvinyl chloride, anodic aluminum oxide (AAO), and rubber.

The batteries of the present invention may be used in downstreamproducts, such as electronic devices, for example, vehicles, laptopcomputers, notebook personal computers, tablet computer, mobile phones(for example, smart phones and the like), personal digital assistants(PDA), display devices (LCD, EL display, electronic paper, and thelike), imaging devices (for example, digital still camera, digital videocamera, and the like, audio devices (for examples, portable audioplayer), a game machine, a cordless handset, an electronic book, anelectronic dictionary, a radio, headphones, a navigation system, amemory card, a pacemaker, a hearing aid, an electric tool, an electricshaver, a refrigerator, an air conditioner, a television, a stereo, awater heater, a microwave oven, a dishwasher, a washing machine, adryer, a lighting device, a toy, a medical device, a robot, a roadconditioner, a traffic light, and the like, but are not limited thereto.

Synthesis and Fabrication Procedures

FIG. 1 shows a schematic diagram for the vapor deposition of gammamonoclinic phase sulfur on carbon nanofibers (CNFs). The CNFs arefabricated using standard methods. The CNFs are then hole punched(diameter 11 mm) and placed in the vapor deposition assembly.

The vapor deposition assembly consists of a stainless steel autoclave asshown in FIG. 2. Other materials commonly used for autoclaves may beemployed as well, for example, aluminum autoclaves. Typically, theautoclave consists of a top part or autoclave lid and bottom part thatdefines an autoclave chamber. The top part forms a lid that covers thechamber of the bottom part. The chamber of the bottom part consists of asulfur reservoir zone and sieve zone. The sulfur reservoir zone of thisparticular apparatus is loaded with about 10.8-11 grams of sulfur(Sigma-Aldrich) in an aluminum boat. The sieve is then placed such thatbubbles of hot melted sulfur do not contact the eight to ten cathodesthat are placed on the sieve, i.e. a distance of about 5 cm ismaintained between the topmost part of a 20 mL aluminum crinkle dishfrom VWR, and the sieve. An O-ring is used to seal the autoclave toprevent escape of sulfur vapor. After heating the autoclave for 25 hoursin an oven at a specified temperature of, for example, 175° C., theautoclave was cooled to ambient temperature of about 18-25° C. in anambient air environment. It is believed that sudden removal of theautoclave from 175° C. to ambient atmosphere results in ahigh-temperature gradient that facilitates the formation of gammamonoclinic phase.

Material Characterization

SEM images show a conformal coating of the gamma monoclinic phase sulfuron the carbon nanofibers, in addition to large chunks of the gammamonoclinic phase sulfur located in the interfiber spacing. The phase ofthe synthesized material was characterized using X-ray diffraction; thepeaks match well with the gamma monoclinic phase sulfur, according tothe Joint Committee on Powder Diffraction Standards (JCPDS) Database.The gamma monoclinic phase sulfur deposited carbon cathodes wereemployed in carbonate-based electrolytes (1M LiPF6 in EC: DEC (1:1 v/v))and showed a single plateau discharge-charge behavior with anexceptional capacity of 800 mAh/g after 1000 cycles at a C/2 cyclingrate with high coulombic efficiency. The current loading is 0.5-0.7 mgof gamma monoclinic phase sulfur, with 30% higher weight loadings ofgamma monoclinic phase sulfur being possible. In addition, thesecathodes are used without additional binders/current collectors furthereliminating dead weight in the battery. Use of the gamma monoclinicphase of sulfur with a non-confinement-based approach enables the use ofa solid state reaction without the need for complex and expensiveconfining architectures and bypasses polysulfide formation, enabling thebattery to achieve a high capacity with an extended cycle life.

A morphological analysis of the vapor deposited sulfur on the CNFs wasconducted using scanning electron microscopy (SEM). The SEM images(FIGS. 3A-3D) show that the sulfur is deposited conformally on theexterior surfaces of CNFs, and that blocks of sulfur are deposited inbetween the fibers in the inter-pore area. To analyze and quantify thematerial, X-ray diffraction was employed. The X-ray diffraction peaks ofthe vapor deposited sulfur matched the pattern of gamma monoclinic phaseof sulfur (JCPDS file from the International Center for DiffractionData) (FIG. 4A). X-ray diffraction also confirmed that the depositedsulfur was a crystalline material that allows for use of the sulfur inthe Li—S battery without confinement. Weight loading of the sulfur onthe CNFs was calculated by thermogravimetric analysis (TGA).Thermogravimetric analysis was carried out in nitrogen from roomtemperature to 600° C. at a heating rate of 10° C./min. The weightloading of the cathodes was about 50% with 0.6-0.7 mg of gammamonoclinic phase sulfur (FIG. 4B).

Electrochemical Characterization

Electrochemical characterization was carried out with gamma monoclinicphase sulfur vapor deposited on CNFs as a positive electrode, a lithiumdisk (diameter 13 mm) as a negative electrode, a celgard separator and a30 microliter electrolyte (1M LiPF₆ in ethylene carbonate:diethylenecarbonate 1:1 v/v) in a CR2032 coin cell. The positive electrode wasfree-standing, had about a 50% weight loading of gamma monoclinic phasesulfur and an absolute amount of gamma monoclinic phase sulfur of about0.6-0.7 mg. To analyze the electrochemical performance, charge-dischargetests were performed at a slow C/50 rate to be able to detect evidenceof side reactions.

Free-standing carbon nanofibers-based electrodes are defined aselectrodes without binder and conductive additives which may be held andused independently of a metallic current collector. These are standaloneelectrodes since they have electron channels originating from carbonnanofibers and may not need an additional metallic sheet for mechanicalstability and electrical conductivity. Depending on specific devicerequirements, one may need to use a current collector sheet.

FIG. 5A shows initial discharge and charge curves at C/50 with acapacity of about 1170 mAh/g and 1169 mAh/g. The curves also display asingle plateau conversion with no evidence of side reactions.

FIG. 5B shows charge-discharge curves at higher current rates, C/10, C/5and C/2. The corresponding discharge capacities are 1100, 1000 and 920mAh/g,m respectively. This electrode demonstrates stable operationalperformance, exhibiting a relatively constant capacity over 1300 cycles.

FIG. 5C shows stable charge-discharge curves at a current rate of C/2for various numbers of cycles.

The cyclic voltammogram of FIG. 5D, conducted at a scan rate of 0.05mV/s, shows a single stable plateau during both the oxidation andreduction cycles corresponding to the charge-discharge curves.

FIG. 5E displays the rate performance of gamma monoclinic phase sulfurvapor deposited on the CNFs to form a cathode. The cathode retainedcapacities of about 800 mAh/g at a rate of C/20, 700 mAh/g at a rate ofC/10, 750 mAh/g at a rate of C/5 and 600 mAh/g at a rate of C/2 for 30cycles each. Further decreasing the current density to C/10, a capacityof 700 mAh/g could be achieved, demonstrating the exceptional ratecapability of the electrode.

The Li—S battery demonstrated ultra-stable and long-life cyclingperformance for 1300 cycles at a current rate of C/2 with a capacity ofabout 700 mAh/g. This remarkable stability can be attributed to thesolid to solid conversion of gamma monoclinic phase sulfur to Li₂S,thereby eliminating formation of polysulfides. The excellent performanceof these gamma monoclinic phase sulfur electrodes demonstrates thefeasibility of high-energy carbonate-based Li—S cells for practicalapplication.

It is considered that Li—S batteries, as well as other sulfur batteries,can be made with gamma monoclinic phase sulfur loadings on carbonnanofibers of from 3-6 mg/cm², or from 4-5 mg/cm².

Free-standing carbon nanofiber/S cathodes were also tested for long-termcycling in room temperature Na—S batteries. The as-prepared CNF/S (˜1.08mg/cm², 43.8 wt % S) cathodes were used directly as cathodes in 2032type coin-sized room temperature Na—S cells. The electrochemical testswere performed in an ethylene carbonate/propylenecarbonate/fluoroethylene carbonate-based electrolyte against an Na/Na⁺anode. The cyclic voltammetry and charge discharge results showed asingle potential for conversion of sulfur to Na₂S. The CNF/S cathodesdelivered a capacity of about 1229 mAh/g during the first cycle withabout a 51.1% coulombic efficiency. The capacity of the room temperatureNa—S cells was stabilized at about 350 mAh/g after the first cycle andwas sustained for up to 300 cycles with only about a 0.03% decay rateper cycle. The postmortem x-ray diffraction results confirmed theconversion of sulfur to Na₂S and vice versa. Further, in situelectrochemical impedance spectroscopy, postmortem x-ray photonelectricspectroscopy (XPS) and galvanostatic intermittent titration technique(GITT) tests were carried out to investigate the underlying conversionmechanism. The preliminary results showed that the observed singleplateau and long-term cycling of room temperature Na—S cells was theconvoluted effect of deposited sulfur and a stable SEI layer on thecathode side.

Examples

The following examples are illustrative, but not limiting of the methodsand compositions of the present disclosure.

The following materials were employed throughout the examples,Polyacrylonitrile (PAN, Mw 150 000 g/mol), N, N-Dimethylformamide (DMF,purity 99.8%), Sulfur (S, purity 99.998% trace metals basis), Ethylenecarbonate (EC, purity≥99%, acid<10 ppm, H2O<10 ppm), Diethyl carbonate(DEC, purity≥99%, acid<10 ppm, H2O<10 ppm) and Lithiumhexafluorophosphate (LiPF6, Purity≥99.99% trace metals basis, batterygrade) were obtained from Sigma Aldrich. All chemicals were used withoutfurther processing.

Synthesis of Carbon Nanofibers (CNFs):

The free-standing carbon nanofibers were made by electrospinningTypically, 10 wt % Polyacrylonitrile, PAN (Mol wt. 150,000, SigmaAldrich) was added to N, N-Dimethylformamide and stirred overnight toform a polymeric blend. This blend was then loaded into a BectonDickinson 5 mL syringe with a Luer lock tip and an 18-gauge stainlesssteel needle (Hamilton Corporation). The syringe with the needle wasconnected to an NE-400 model syringe pump (New Era Pump Systems, Inc.)in order to control the feeding rate of the solution. The groundedaluminum collector was placed 6 inches from the tip of the needle.Electrospinning was performed at room temperature with relative humiditybelow 15%. A potential difference of 7-8 KV (Series ES (30 KV), GammaHigh Voltage Research, Inc.) was applied between the collector and thetip of the needle. The flow rate of the solution was kept constant at0.2 mL/hr. The as-spun nanofibers were collected and stabilized in aconvection oven at 280° C. for 6 hours in the air. Finally, thesestabilized nanofiber mats were activated by carbonizing in nitrogen uptill 900° C. and dwelled for 1 hour with CO₂ flow in a horizontal tubefurnace. A heating ramp rate of 2° C./min was employed for both thestabilization and carbonization process.

Monoclinic Gamma Phase Sulfur Deposition on Carbon Nanofibers (CNFs):

The free-standing CNFs mat was punched with stainless steel die (ϕ=11mm) and weighed. The CNFs discs were then placed in a house-madeautoclave (Stainless steel 316) and subjected to 180° C. for 24 hours inan oven. The autoclave included a sulfur reservoir at the bottom andperforated disk for placing electrodes at the top. A gap of 1.5 cmbetween the surface of reservoir and the cathodes was used. The distancebetween the reservoir and the electrodes was found to play a key role inthe appropriate loading and deposition of sulfur. After 24 hours theautoclave was cooled to room temperature slowly in a span of 6-8 hours.The heat treatment results in the formation of pungent odors and hencethe autoclave was opened in the fume hood and the electrodes were takenout. The electrodes were weighed and transferred in Argon filled glovebox via overnight vacuum drying in the antechamber, for batteryfabrication.

Characterization Material

Morphological and elemental characterizations of the nanofibers weredone using a scanning electron microscope (SEM) (Zeiss Supra 50VP,Germany) equipped with energy-dispersive X-ray spectroscopy (EDS) withan in-lens detector and 30 μm aperture was used to examine themorphology and obtain micrographs of the samples. X-ray diffraction(XRD) patterns were acquired on a diffractometer (Rigaku Smartlab,Tokyo, Japan) using Cu K_(α) radiation (40 kV and 44 mA) with a stepsize of 0.02° in the 20 range of 10-70°. The surface chemistry of thesamples was analyzed by using XPS spectra (Physical Electronics VersaProbe 5000 spectrometer with monochromatic Al Kα as an excitationsource), with a spot size of 200 μm and pass energy of 23.5 eV were usedto irradiate the sample surface. A step size of 0.5 eV was used togather the high-resolution spectra. CasaXPS Version 2.3.19PR1.0 softwarewas used for spectra analysis. The XPS spectra were calibrated bysetting the valence edge to zero, which was calculated by fitting thevalence edge with a step-down function and setting the intersection to 0eV. The background was determined using the Shirley algorithm which is abuilt-in function in the CasaXPS software. Thermogravimetric analysis(TGA) data of all of the samples were collected on a TA Instruments 2950(TA Instruments, New Castle, Del.) under a steady argon flow at aheating ramp rate of 5° C. min-1. Nitrogen adsorption-desorptionanalysis of the freestanding nanofiber mats was performed at 77 K on anautomated gas sorption analyzer (AutoSorb iQ2, QuantachromeInstruments). The sample was degassed overnight at 150° C. under N₂ flowprior to this analysis.

Electrochemical

Electrochemical measurements were carried out by preparing 2032—typecoin cells (MTI and Xiamen TMAX battery equipment) assembled in anargon-filled glove box (MBraun Labstar pro, MB 10G, H2O and O2<1 ppm).As transferred electrodes were used as working electrodes and 13 mmlithium discs punched from Lithium foil (Alfa Aesar, 75 mm thick) wereused as counter/reference electrodes. To improve the mass loading,cathodes were stacked on to each other. A typical weight loading ofaround 45-50% was used with a mass loading of around 0.5-5 mg/cm² forelectrochemical testing. The ether electrolyte was prepared bydissolving 1.8 M lithium trifluoromethanesulfonate in a solvent mixtureof 1,2-dioxolane (99.8%, anhydrous, stabilized with 75 ppm BHT,AcroSeal) and 1,2-dimethoxyethane (Extra Dry≥99%, AcroSeal) in 1:1volume ratio. The carbonate electrolyte consisted of 1 M LiPF6 in 1:1volume ratio of Ethylene carbonate and Diethyl carbonate (purity≥99%,acid<10 ppm, H2O<10 ppm). Polypropylene membrane (25 μm, Celgard Inc)was used as a separator. The galvanostatic charge-discharge measurementswere carried out in a potential range of 1.0-3.0 V vs Li/Li′ using abattery cycler (Maccor 4000 and Neware BTS 4000). The CV measurementswere performed in a potential range of 1.0-3.0 V vs. Li/Li⁺ at a rangeof scan rates from 0.01-0.5 mV/s using a multi-channel battery tester(Biologic VMP3). Electrochemical impedance spectroscopy (EIS)measurements were performed between 100 mHz to 100 MHz frequency rangeusing an AC perturbation of 10 mV rms amplitude (Biologic VMP3).

Results and Discussion Material Characterization

FIG. 7 provides a schematic outline of a Li—S cell with a gammamonoclinic phase sulfur based cathode in carbonate electrolyte. Briefly,activated carbon nanofibers (CNFs) were prepared by electrospinning 10wt % polyacrylonitrile (PAN) in DMF. The as-spun mats were thensubjected to air stabilization followed by carbonization in an inertenvironment and CO₂ treatment for 1 hour to form free-standing carbonnanofibers. The scanning electron microscopy (SEM) images in FIG. 8Ashow a smooth carbon nanofiber surface with an average diameter of ˜150nm. After sulfur deposition in the autoclave, SEM images reveal aconsistently rough fiber morphology suggesting uniform and conformalcoating of sulfur (FIG. 8B). Few regions display blocks of sulfurdeposited within the inter-fiber spacing. Overall these images provideclear evidence that the sulfur is largely on the outer carbon nanofibersurface.

To further understand the effect of sulfur deposition on surface areaand pore sizes of CNFs, Brunauer-Emmett-Teller (BET) surface areaanalysis was conducted. FIGS. 8D-8E show the N₂ absorption/desorptionisotherm plots and pore size distribution of CNFs before and aftersulfur deposition. For CNFs, the gas uptake increases to a high value ata low relative pressure (P/P₀<0.05), and the adsorption isothermexhibits a plateau at middle and high relative pressures. The hysteresisloop at P/P0=0.2-1.0 represents mesoporosity. The adsorption isotherm isa combination of IUPAC types I & IV isotherms which confirms thepresence of both micro and mesopores on CNFs²¹. However, after thesulfur deposition in the autoclave, we observe the change of theisotherm suggesting a decrease in surface pores with significantly lowergas uptake. Pore size distribution in FIG. 8E denotes CNFs portray amulti-modal pore structure in the nanoscale regime with an average poresize of 2.916 nm and a pore volume of 0.409 cm³/g. After the sulfurdeposition, the CNFs display an enormous reduction in pore volume (0.054cm³/g) suggesting pore filling by sulfur. Pore structural parameters ofall the materials are summarized in Table 1. The BET and SEM datasuggest that sulfur is partially confined within the carbon nanopores.Nevertheless, there is clear evidence of exposed unconfined sulfur onthe external carbon surface which differentiates the structure of thepresent invention from prior art structures where the deposited sulfuris confined within the pores of the material. For example, at least 2 wt%, or at least 5 wt % or at least 10 wt % or at least 20 wt % of thesulfur is deposited on the surface. FIG. 8F shows the thermogravimetricanalysis on sulfur-deposited CNFs conducted in inert nitrogen atmospherewith a heating rate of 10° C./min from room temperature to 600° C. TheTGA curve shows mild initial weight loss below 100° C. associated withevaporation of adsorbed moisture. A continuous weight loss beyond thispoint was observed over a wide temperature window until 300° C. with twodistinct degradation rates. The melting of sulfur occurs at 119° C. andsulfur starts to evaporate soon after due to its high vapor pressure.The wide decomposition temperature range and multiple degradation ratesobserved in TGA further corroborate partial pore-confinement of sulfur.While the higher-rate lower-temperature weight loss suggests evaporationof exposed unconfined sulfur, the lower-rate higher-temperature loss canbe attributed to sulfur confined in micro/mesopores. The sulfur contentin the CNFs/S composite determined by TGA was 50 wt %.

TABLE 1 Average pore S_(BET) V_(t) V_(mic) diameter Sample (m²/g)(cm³/g) (cm³/g) (nm) CNFs before 558.4 0.2712 0.215 2.916 thermaltreatment CNFs after 31.78 0.02603 0.00542 6.821 thermal treatment

FIG. 9A shows the room temperature XRD patterns of bare CNFs and aftersulfur deposition. The bare CNFs display no significant diffractionpeaks. A wide hump is seen from 2 theta of 20-30 degrees due to theamorphous nature of carbon in CNFs. However, after the sulfurdeposition, we see a rare and metastable phase of sulfur—the monoclinicgamma phase (Rosykite). This is striking behavior since onlyorthorhombic-alpha (S8), rhombohedral (S6), hexagonal (S8) and polymericallotropes of sulfur are known to be stable at room temperature²⁴.Nevertheless, repeatable XRD signatures after sulfur depositiontreatment show the presence of gamma-monoclinic sulfur in the samples.It is established in the literature that the monoclinic phases (largelyα and β) are stable only at temperatures>95° C.^(22,23) and below thistemperature the monoclinic phase quickly converts back to the stableorthorhombic form. Only a handful of reports in the past two centurieshave even mentioned the presence of gamma-monoclinic sulfur at roomtemperature for short periods²⁵⁻²⁷.

However, the present gamma-monoclinic phase has been shown to be stablefor at least 395 days at room temperature and it remains stable, showingno signs of phase change. A recent DFT study on stabilization ofmetastable sulfur shows that the carbon host can facilitate thestabilization of a monoclinic sulfur phase if the number of carbon atomsexceeds 0.3 per S8 unit crystal structure²⁸. In addition, it wasrecently suggested by Moon et al. that carbon facilitates the formationand helps in retaining the monoclinic structure at room temperature forlonger periods²⁵. In our study, we hypothesize that the monoclinic gammaphase formed at elevated temperatures penetrates the porous carbonstructures and retains its crystal structure even after cooling due tothe local carbon density within the pores. This unique crystal structureonce trapped within the pores possibly propagates throughout the sulfurblocks including those that are externally deposited in an “unconfined”state.

FIGS. 9B and 9C show the EDX mapping and corresponding low magnificationSEM image exhibiting uniform distribution of sulfur. To confirm thechemical composition and surface properties of pristinegamma-monoclinic-sulfur based CNF cathodes (gamma sulfur depositedCNFs), XPS measurements were performed and the results are displayed inFIGS. 9D-9F. The survey spectra, shown in FIG. 17, shows the existenceof C1s, S2p, and O1s peaks in the composite. The peaks centered at 284.6eV, 531.0 eV, and 533 eV correspond to the C1s, O1s and the adsorbedsurface hydroxyl group (—OH), respectively²⁹. FIG. 9D shows thehigh-resolution S2p spectra of the composite. The S2p_(3/2) peak at163.7 eV and S2p_(1/2) peak at 164.9 eV with an area ratio of 1:2 anddelta E of 1.18 eV are the characteristic peaks of solid sulfur in thecomposite³⁰. Another broad peak centered at 168.8 eV can be attributedto the surface oxidation of sulfur during high-temperature sulfurdeposition treatment. The smooth Lorentzian asymmetric peak of carbonfurther confirms that sulfur does not react with the bare carbonsurface.

Electrochemical Characterization

FIG. 10 shows the electrochemical performance evaluation of gamma sulfurdeposited CNFs used as free-standing cathodes in CR 2032 type coin cellswith reference/counter as lithium. Both ether- and carbonate-basedelectrolytes were employed to understand the electrochemical phenomenonin each system. The electrode demonstrates a reversible electrochemicalredox behavior in both ether and carbonate-based electrolytes. However,the charge-discharge profiles are drastically different. (FIG. 4a ). Theether-based charge-discharge profile exhibits a standard two-plateaubehavior as reported in most prior literature reports. The first plateauat 2.3 V is attributed to the conversion of sulfur to long-chainpolysulfides and the second plateau at 2.1 V represents the conversionof long-chain polysulfides to Li₂S₂ and Li₂S (2.1 V). However, the samegamma sulfur deposited CNF cathodes in carbonate electrolytesdemonstrate a single plateau at 2.0 V in the first and all consecutivecycles during discharge and 2.2 V in charge profiles suggesting thepossibility of a polysulfide digression route to directly form lithiumsulfide in carbonate electrolyte. This solid-to-solid conversionpossibly also leads to a higher overpotential explaining the lowerplateau voltage observed in the carbonate electrolyte. Theelectrochemical behavior is consistent with CV profiles, wherein thecells with ether electrolyte show two peaks, while the cells withcarbonate electrolyte only show a single peak.

FIG. 10C shows the long-term cycling performance tested under variousgalvanostatic modes. The discharge capacities were calculated at bothlow and high current rates of 0.1 C and 0.5 C and the capacitiesretained after 1000 cycles were 500 mAh/g and 770 mAh/g, respectively.The results for the discharge capacities at both low current rate (0.1C) and high current rates (0.5 C), are shown in FIGS. 18A and 18B,respectively. FIG. 10C shows the long-term cycling data for cells madeusing carbonate electrolyte. These cells demonstrate ultra-stable andprolonged cycling at 0.5 C rate for 2000 cycles with only 0.113% decayafter the initial 200 cycles. The cells retained a capacity of 704 mAh/geven after 2000 charge-discharge cycles. The initial drop in capacitymay be attributed to the loss of contact due to volume expansion duringcycling which stabilizes as the cycling proceeds. As shown in FIG. 10D,the discharge profile continues to exhibit a single plateau through theentire cycle life of 2000 cycles.

It has been established in a recent report by Kim et al. thatpolysulfides, if generated, attack carbonate species via nucleophilicsubstitution to form the irreversible products, thiocarbonate andethylene glycol, and shut down further electrochemical activity afterthe first cycle. Therefore, our data suggests that these gamma sulfurdeposited CNF-based cells continue to follow a polysulfide digressionroute through the entire cycling in carbonate electrolyte, whichexplains not only continued battery operation despite the presence ofcarbonate species but also the excellent cycle stability of 2000+cycles. For comparison, the cycling data in ether electrolyte is shownwherein these materials follow a standard route with polysulfides as theintermediate products. The cycling data in ether electrolyte may befound in FIG. 19. Here we see a gradual decline in capacity due to theexpected polysulfide shuttling and subsequent loss of active material.

To further corroborate this unique electrochemical behavior of gammamonoclinic phase sulfur in carbonate electrolyte and infer informationabout the reaction mechanism, differential capacity (dQ/dV) analysis andEIS as a function of voltage were conducted. A consistent single peak isseen throughout the 2000 cycles further supporting a single-phaseconversion. The peaks minimally shift during the cycling suggesting goodmaterial integrity and only a minimal increase in resistance duringcycling. As a next step, the electrochemical impedance spectroscopy(EIS) measurements of the lithium half-cells with gamma sulfur depositedCNFs as composite cathodes were carried out at various potentials duringcharge-discharge cycles. FIG. 10F presents the typical Nyquist plots forthe Li—S batteries of the invention illustrating their impedance trendsas a function of voltage. As seen in this figure, a typical Nyquist plotconsists of a semicircle in the high frequency to medium frequencyrange, which is attributed to the interfacial charge transferresistance. The charge transfer resistance (Ret) monotonically decreasesas the cathodic curve progresses towards a lower potential for theentire discharge cycle. The trend is reversed when the battery ischarged back to a higher potential. This observation contrasts with theliterature, wherein the R_(ct) is shown to first decrease and thenincrease back again in the same discharge cycle suggesting the formationof soluble polysulfides at intermediate voltages. These intermediatepolysulfides significantly lower the R_(ct) due to disappearance of bothof the solid insulating materials—the initial reactant, sulfur and finalproduct, Li₂S. In the literature, the R_(ct) of the final dischargedcell still remains lower than the initial R_(ct) (at OCV) due to reducedresistance of Li₂S compared to pure sulfur. A monotonic decrease inR_(ct) during discharge in the present experiments provides furtherevidence that we are eliminating the formation of polysulfides.

To evaluate the practical application of the carbonate-based Li—Ssystem, the cells were cycled with gamma sulfur deposited-CNFs cathodesat various C rates and loadings. As shown earlier, these batteriesdemonstrate stable capacity at a 0.5 C rate for over 2000 cycles. Todemonstrate battery operation at harsh conditions, the batteries weretested for long term cycling at 0.1 C. The batteries provided stable˜600 mAh/g capacity for over 1000 cycles with a small 0 0015% decay anda coulombic efficiency>=99%. In addition, these batteries show excellentrate performance with capacities of 1170, 1080, 980, 900, 750, 600 and410 mAh/g at 1 C, 2 C, 5 C, 10 C, 15 C, 30 C and 40 C, respectively. Itis interesting to see these cells exhibiting a capacity of 400 mAh/geven at 40 C corresponding to a discharge and charge time of only ˜30seconds. The traditional ether-based batteries perform only up to 2 C atwhich the performance deteriorates significantly. FIG. 11B shows thatthe cells exhibit a similar single plateau discharge at all C rates.Such rate capability suggests efficient nanoscale contact between thegamma-monoclinic sulfur and the host CNFs and good interfacialelectrode-electrolyte contact owing to the 3D inter-fiber porousarchitecture. Furthermore, the binder-free freestanding format of theCNF host appears to provide uninterrupted electron pathways despite thepresence of insulating sulfur. This is unique compared to traditionalslurry-based cathodes where carbon and sulfur powders are mixed togetherwith limited to no control over spatial morphology which therebydeteriorates overall composite conductivity. FIG. 11B shows the cyclingdata for higher commercially-relevant sulfur loadings. Cells with 5mg/cm² of sulfur demonstrate stable cycling for 300+ cycles at 0.1 C.This finding demonstrates that unconfined sulfur deposition using gammamonoclinic phase sulfur on external carbon surfaces make it possible toprovide commercially-relevant sulfur loadings in carbonate electrolyte.

A handful of reports in the literature have reported single plateaudischarge in carbonate electrolyte in cathodes where sulfur was shown tobe fully confined in sub-nano pores. Such discharge behavior has beenexplained via confinement-driven hypotheses, one example being that thesub-nanometer pore sizes limit the sulfur chain length during synthesisresulting in the formation of smaller sulfur allotropes (S₂—S₄), whichare then directly converted to Li₂S during discharge eliminatinglong-chain polysulfides. Another hypothesis that has been put forth inthe literature is that the confinement of Ss molecules in sub-nanometerpores forces de-solvation of the Li-ions and leads to solid-statelithiation/de-lithiation preventing adverse reactions betweenpolysulfides and carbonate species [Fu and Xin]. These works relied oncomplete nano-confinement of sulfur. In contrast, in the cathodes of thepresent invention, sulfur is largely present on the external carbonsurface. Exposed unconfined sulfur of this type has been previouslyassociated with irreversible reactions with carbonate electrolyte andbattery shut down after the first cycle as shown by Kim et al. Onestriking difference of the present invention is the crystal structure ofthe sulfur in the cathodes. Most Li—S literature, regardless of theelectrolyte, uses α-orthorhombic sulfur, which is the most stable sulfurallotrope at room temperature. It is therefore likely that the singleplateau behavior seen reversibly and consistently for 2000 cycles in thepresent invention is directly linked to the role of gamma-monoclinicsulfur phase.

A possible reason for such a significant effect of sulfur crystalstructure could be the difference in phase density. While there arediscrepancies in the reports on densities of various sulfur allotropesas synthesizing a metastable allotrope is non-trivial, Meyer et al. didground breaking work on sulfur allotropes in the early 1960's. Hereported the density for gamma-monoclinic-sulfur as being higher thanits α-counterpart (2.19 g/cm³ vs 2.069 g/cm³). The close compactnesswithin the gamma-monoclinic crystal structure possibly provides greaterstability and easy lithiation into the gamma monoclinic crystalstructure in carbonate electrolyte. In the ether electrolyte, it isbelieved that gamma monoclinic phase sulfur converts to a more favorablephase to yield a two-plateau discharge. Study on stability of thisunique sulfur crystal structure in various electrolytes is underway.

Post mortem studies were conducted using XRD and XPS to understand theredox products after charge and discharge cycles and to provide evidencethat the stable capacity is indeed largely a result of the desiredsulfur to Li₂S reactions (and not any unwanted degradation reactions).This is also particularly important as most papers reporting singleplateau discharge profile in Li—S batteries do not provide reactantand/or product characterization for deeper understanding of the chargestorage mechanism and to evaluate electrolyte decomposition (if any).Below we discuss both post mortem spectroscopy and microscopy data.

Postmortem SEM and TEM Analysis

To understand chemistry and surface morphology after harsh cyclingconditions, we conducted post mortem microscopy of cycled cells. Thesurface morphology gamma sulfur deposited-CNFs after 20 charge anddischarge cycles at C/20 is shown in FIGS. 12A-12B. Compared to pristinesamples, the charged and discharged samples still retain theirfreestanding architecture. However, the surface deposited gammamonoclinic phase sulfur redistributes itself on the surface possibly dueto volume expansion-contraction during discharge-charge cycles.Nevertheless, gamma monoclinic phase sulfur still remains exposed andunconfined on the surface of CNFs

FIG. 12C shows a TEM image taken after lithiation in a completelydischarged sample post 1000 cycles showing the adherence of sulfur tocarbon nanofibers. Furthermore, the preserved free-standing gamma sulfurdeposited-CNFs architecture in this TEM image corroborates with theexcellent cycling performance implying that the electrode structure canafford to accommodate the volume expansion of sulfur during cycling andalso facilitate better electrode-electrolyte contact throughout thecycle life.

Postmortem XPS and XRD Analysis

FIGS. 13A-13I provide the post mortem XPS and XRD data both afterdischarge and after charge. FIG. 13D shows typical charge-dischargevoltage profiles obtained for the samples and the specific points wherespectroscopy data was collected after discharge and after charge. Priorto XPS analysis, the cycled samples were thoroughly rinsed with theelectrolyte solvent and dried out under Ar atmosphere and later underdynamic vacuum for 48 hours. The samples were then loaded in an XPStransfer assembly in the glove box and transferred to an XPS vacuumchamber avoiding any contact with ambient atmosphere. A pass energy of23.5 eV, with a step size of 0.5 eV was used to gather thehigh-resolution spectra. CasaXPS Version 2.3.19 PR1.0 software was usedfor peak fitting. The XPS spectra were calibrated by setting the valenceedge to zero, which was calculated by fitting the valence edge with astep-down function and setting the intersection to 0 eV. The conductiveelements were fitted using an asymmetric Lorentzian line shape. Thenon-conductive peaks, on the other hand, were fitted using an asymmetricGaussian/Lorentzian line shape. The background was determined using theShirley algorithm, which is a built-in function in the CasaXPS software.

As discussed earlier, in the pristine sample with vapor deposited gammamonoclinic phase sulfur, we see the presence of adventitious carbon, Cat 284.6 eV from the CNF surface. The S2P spectra shows the presence ofsulfur doublet peaks (S2p_(3/2) and S2p_(1/2)) positioned at 163.7 and164.9 eV with a peak separation of 1.18 eV. In addition, we see a peakat higher binding energy (168.94 eV) associated with the formation ofsurface oxides (S—O) during high temperature deposition. Similar bondscan be seen in the O1s spectra, wherein the peak at 531.86 eV can beattributed to the surface oxide. Extreme care was taken during transferof the samples, hence another peak at 532.34 eV was ascribed to asurface oxidized SOx group.

After complete discharge, the S_(2p) spectra shows the appearance of anew strong peak at a lower binding energy of 161.8 eV associated withlithium sulfide (Li₂S) deposition. Interestingly, the presence of a newpeak at 685.5 eV is attributed to LiF in the F1s spectra. In addition,we see a diminished peak contribution from surface deposited electrolytesalts (LiFxNy) at 688 eV compared to the charged sample spectra. Thesignature of LiF species was not seen in the postmortem XRD spectrum (tobe discussed below) denoting its extremely low contribution. PostmortemSEM or TEM images of charged and discharged samples shown above did notdemonstrate formation of spherical agglomerates on the surface of CNFsoften associated with LiF formation, further suggesting low LiFdeposition originating from specific sites.

The carbonate electrolyte is expected to be stable and not decompose inthe 1-4.2 V vs Li/Li+ range and the formation of LiF can be attributedto decomposition of the salt and not the organic electrolyte as noorganic species can be seen in the cycled C1s and O1s spectra. Also, atlower scan rates no current response was recorded corresponding topossible salt decomposition.

After complete charge, S2p spectra shows a diminished Li₂S peak at 161.8eV and sulfur doublet peaks dominate the overall spectrum indicatingreversible conversion of Li₂S back to sulfur in the charge cycle. Thepresence of some Li₂S even after charge may be associated withincomplete conversion due to the fast charge rate. The F1s spectracontinues to show the presence of LiF and LiFxNy peaks from saltdecomposition and salt species at 685.5 and 688 eV, respectively, incharged samples, which is expected as LiF is known to be a stable SEIcomponent in Li-ion batteries.

FIGS. 13A-13C show the XRD patterns of the pristine gamma sulfurdeposited-CNFs, as well as discharged and charged cathodes. Thediffractogram of the pristine cathode as discussed earlier shows peaksof gamma monoclinic phase sulfur. During initial discharge, we observe aplateau at 2.0 V. The plateau continues to progress towards completesulfur reduction at 1.0 V at a rate of 0.1 C. After complete reductionof cathodes, the diffraction pattern shows the presence of Li₂S peaks(00-023-0369) at 11.61, 13.41, 19.01, 22.31, which correspond toreflections (111), (200), (220) and (311), respectively. It confirms thefinding from XPS and TEM that the single plateau observed in thedischarge cycle is associated with the reduction of gamma monoclinicphase sulfur to lithium sulfide (Li₂S). After charge, interestingly, acompletely different sulfur XRD pattern is observed. Such pattern hasnot been reported yet in the Li—S literature. These peaks are attributedto cyclo-deca-cyclo-hexa sulfur, also belonging to a monoclinic crystalstructure family. No overlapping gamma monoclinic sulfur peaks wereobserved. This post mortem study demonstrates complete conversion ofβ-monoclinic sulfur to Li₂S and back to a new sulfur monoclinic crystalphase. This is the first ever study to report stability of such sulfurcrystal structures in Li—S batteries and their operation in carbonateelectrolyte. It has been previously reported in ether-based Li—Sbatteries that α-orthorhombic sulfur allotrope (the most stable sulfurallotrope at room temperature) indeed converts to the β-monoclinic phaseand that phase dominates after the first charge cycle. Using thisanalogy, we hypothesize that the monoclinic phase is thermodynamicallymore stable in the Li—S electrolyte medium and is therefore retained inour system even after the charging cycle. Nevertheless, we observe aunique monoclinic phase in these charged samples, which is differentfrom the β-monoclinic phase seen in ether electrolyte and this possiblyplays a role in retaining single plateau behavior in charge-dischargeprofiles for over 2000 cycles. All experiments with Li—S cells werecarried out at about 18-25° C. unless otherwise specified.

Sodium Sulfur Cells FIG. 14A displays a cyclic voltammetry curve ofgamma sulfur deposited-CNFs cathode with a sodium anode in 1M NaClO₄ inEC:DEC (1:1 V/V) with 5% FEC at various scan rates. In the reductioncurve, two peaks can be seen which are attributed to the conversion ofsulfur to short chain sodium sulfides Na₂S₂ and Na₂S, respectively.Further, a similar trend in the oxidation curve is seen where Na₂Sconverts to Na₂S₂, and then finally to sulfur. FIG. 14B displays therate performance of gamma sulfur deposited-CNFs in carbonate-basedelectrolytes. It delivers a high initial capacity of 780 mAh/g at a 0.05C rate. Further improvement in the C rate to 0.1 C and 0.2 C, causes aloss of capacity attributed to higher current densities and lower timefor conversion and reaction, which capacities stabilized at 550 mAh/gand 325 mAh/g, respectively. These experiments were carried out at about18-25° C.

Potassium Sulfur Cells

FIG. 15A displays a cyclic voltammetry curve of a gamma sulfurdeposited-CNFs cathode with a potassium anode in 1 M KPF₆ in EC:DEC atvarious cycles at 0.1 mV/s. In the reduction peak, 3 distinct peaks areattributed to the conversion of sulfur to K₂S₃ to K₂S₂ and finally toK₂S. However, during the charge step, a single oxidation peak isobserved, which is attributed to direct conversion to sulfur withoutintermediate conversion. A gamma sulfur deposited-CNFs cathode performssubstantially better than any cathodes reported in the literature up to5 C. The cathode retains all of its capacity when cycled back at 0.1 C,displaying superior cathode architecture and complete utilization of theactive material. Cycling stability was studied at 0.5 C rate, and thecathode displayed a capacity of 870 mAh/g after 120 cycles showing itspotential to be used as a cathode material in a potassium-sulfur system.These experiments were carried out at about 18-25° C.

Other embodiments of the present disclosure will be apparent to thoseskilled in the art from consideration of the specification and practiceof the embodiments disclosed herein.

As used throughout the specification and claims, “a” and/or “an” mayrefer to one or more than one. Unless otherwise indicated, all numbersexpressing quantities of ingredients, properties such as molecularweight, percent, ratio, reaction conditions, and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about,” whether or not the term “about” ispresent. Accordingly, unless indicated to the contrary, the numericalparameters set forth in the specification and claims are approximationsthat may vary depending upon the desired properties sought to beobtained by the present disclosure. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. Notwithstanding that thenumerical ranges and parameters setting forth the broad scope of thedisclosure are approximations, the numerical values set forth in thespecific examples are reported as precisely as possible. Any numericalvalue, however, inherently contains certain errors necessarily resultingfrom the standard deviation found in their respective testingmeasurements. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of thedisclosure being indicated by the following claims.

The foregoing embodiments are susceptible to considerable variation inpractice. Accordingly, the embodiments are not intended to be limited tothe specific exemplifications set forth hereinabove. Rather, theforegoing embodiments are within the spirit and scope of the appendedclaims, including the equivalents thereof available as a matter of law.Other suitable modifications and adaptations of the variety ofconditions and parameters normally encountered in the field, and whichare obvious to those skilled in the art, are within the scope of thedisclosure.

All patents and publications cited herein are fully incorporated byreference herein in their entirety or at least for the portion of theirdescription for which they are specifically cited or relied upon in thepresent description.

The patentees do not intend to dedicate any disclosed embodiments to thepublic, and to the extent any disclosed modifications or alterations maynot literally fall within the scope of the claims, they are consideredto be part hereof under the doctrine of equivalents.

It is to be understood that each component, compound, substituent orparameter disclosed herein is to be interpreted as being disclosed foruse alone or in combination with one or more of each and every othercomponent, compound, substituent or parameter disclosed herein.

It is also to be understood that each amount/value or range ofamounts/values for each component, compound, substituent or parameterdisclosed herein is to be interpreted as also being disclosed incombination with each amount/value or range of amounts/values disclosedfor any other component(s), compounds(s), substituent(s) or parameter(s)disclosed herein and that any combination of amounts/values or ranges ofamounts/values for two or more component(s), compounds(s),substituent(s) or parameters disclosed herein are thus also disclosed incombination with each other for the purposes of this description.

It is further understood that each range disclosed herein is to beinterpreted as a disclosure of each specific value within the disclosedrange that has the same number of significant digits. Thus, a range offrom 1-4 is to be interpreted as an express disclosure of the values 1,2, 3 and 4.

It is further understood that each lower limit of each range disclosedherein is to be interpreted as disclosed in combination with each upperlimit of each range and each specific value within each range disclosedherein for the same component, compounds, substituent or parameter.Thus, this disclosure to be interpreted as a disclosure of all rangesderived by combining each lower limit of each range with each upperlimit of each range or with each specific value within each range, or bycombining each upper limit of each range with each specific value withineach range.

Furthermore, specific amounts/values of a component, compound,substituent or parameter disclosed in the description or an example isto be interpreted as a disclosure of either a lower or an upper limit ofa range and thus can be combined with any other lower or upper limit ofa range or specific amount/value for the same component, compound,substituent or parameter disclosed elsewhere in the application to forma range for that component, compound, substituent or parameter.

REFERENCES

The following references may be useful in understanding some of theprinciples discussed herein:

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1. A method of depositing monoclinic sulfur on a substrate, comprisingsteps of: depositing monoclinic phase sulfur that is stable at atemperature below 80° C. via vapor deposition onto a substrate in asealed vapor deposition apparatus at a temperature sufficient tovaporize sulfur and for a time sufficient to provide the monoclinicphase sulfur, and cooling the monoclinic phase sulfur deposited on theporous material to a temperature of 0-50° C.
 2. The method of claim 1,wherein the method provides a substrate with monoclinic sulfur depositedtherein that is suitable for use as a cathode in an electrode of a cellor battery and a loading of the monoclinic phase sulfur on the substrateis at least 0.05 mg/cm².
 3. The method of claim 1, wherein thedepositing step is carried out in the sealed vapor deposition apparatusat a temperature of from 140° C. to 350° C.
 4. The method of claim 1,wherein the depositing step is carried out for a period of at leastabout 1 minute.
 5. The method of claim 1, wherein the cooling step iscarried out by exposing the vapor deposition apparatus or the cathode toa temperature of less than 50° C.
 6. The method of claim 5, wherein thevapor deposition apparatus is cooled immediately upon completion of thedepositing step.
 7. The method of claim 1, wherein the substrate is aporous material which has a pore volume of 10-95% after deposition ofthe monoclinic phase sulfur on the substrate.
 8. The method of claim 1,wherein the substrate is conductive.
 9. The method of claim 1, whereinthe substrate is non-conductive.
 10. The method of claim 1, wherein thesubstrate comprises carbon nanofibers.
 11. The method of claim 10,wherein the carbon nanofibers onto which the monoclinic phase sulfur isdeposited are free-standing carbon nanofibers.
 12. The method of claim1, wherein the substrate is positioned in the vapor deposition apparatuswith a gap between a surface of a liquid sulfur reservoir and thesubstrate such that the substrate does not come into contact with liquidsulfur during the vapor deposition step.
 13. The method of claim 12,wherein the monoclinic phase sulfur comprises a form of sulfur selectedfrom the group consisting of monoclinic gamma phase sulfur; monoclinicsulfur that best matches monoclinic gamma phase sulfur using PDXLVersion 2.8.4.0 Integrated Powder Diffraction Software; monoclinic gammaphase sulfur oriented in the k direction; and monoclinic sulfur thatbest matches monoclinic gamma phase sulfur oriented in the k directionusing PDXL Version 2.8.4.0 Integrated Powder Diffraction Software.14-35. (canceled)
 36. The method of claim 1, wherein the method providesa substrate with monoclinic sulfur deposited therein that is suitablefor use as a cathode in an electrode of a cell or battery and a loadingof the monoclinic phase sulfur on the substrate is at least 0.5 mg/cm².37. The method of claim 1, wherein the depositing step is carried out inthe sealed vapor deposition apparatus at a temperature of from 150° C.to 210° C.
 38. The method of claim 1, wherein the depositing step iscarried out for a period of 10 to 30 hours.
 39. The method of claim 1,wherein the cooling step is carried out by exposing the vapor depositionapparatus or the cathode to a temperature of from about −20° C. to about25° C.
 40. The method of claim 1, wherein the method provides asubstrate with monoclinic sulfur deposited therein that is suitable foruse as a cathode in an electrode of a cell or battery and a loading ofthe monoclinic phase sulfur on the substrate is at least 0.5 mg/cm² andthe depositing step is carried out in the sealed vapor depositionapparatus at a temperature of from 150° C. to 210° C. for a period of 10to 30 hours.
 41. The method of claim 40, wherein the cooling step iscarried out by exposing the vapor deposition apparatus or the cathode toa temperature of from about −20° C. to about 25° C. immediately uponcompletion of the depositing step.
 42. The method of claim 1, whereinthe monoclinic phase sulfur comprises a form of sulfur selected from thegroup consisting of monoclinic gamma phase sulfur; monoclinic sulfurthat best matches PDF Card No.: 00-013-0141 Quality:B for Rosickyite,monoclinic gamma phase sulfur, using PDXL Version 2 Integrated PowderDiffraction Software.